This invention relates to the field of liquid ink printing systems and, more particularly, to a method for improved print quality and color density in color ink jet printing systems.
A typical ink jet printing system includes a platen for supporting a substrate such as paper on which an image is to be printed. A drive means such as a motor is provided for advancing the platen and thereby advancing the paper. A print head having one or more nozzles for ejecting drops of ink or a similar liquid printing solution is mounted on a motor-driven carriage. The carriage is moveable along a path transverse to the paper advancement path, and supports the print head with the nozzle(s) facing the platen in an adjacent yet slightly spaced-apart relationship. Platen (paper) position and carriage location combine to position the print head opposite a desired location on the paper.
Ink jet print heads include at least one and often a plurality of print nozzles. In the latter case, the print nozzles typically form a linear array, arranged vertically with respect to the printing medium (i.e., along a line parallel to the direction of paper advancement). For color ink jet printing, the print head typically includes nozzles for ejecting cyan, magenta and yellow colored ink, called the primary printing colors, or simply "primaries." Some systems additionally include nozzles for ejecting black ink. A print head array may also include several nozzles for ejecting each color of ink. In a preferred embodiment described below, the print head includes sixteen nozzles for each of the three (C, M and Y) primaries. A controller, for example, a microprocessor system including associated memory and interfacing electronics, controls the platen drive means, carriage motor and print head.
Printing occurs as the print head traverses across the width of the paper (a "pass"). During each pass all sections of the head are printing, each section printing on a different horizontal band of the paper. Between passes of the print head, the paper is advanced a distance equal to the height of one color section of the head. Paper advances past the printing head from the bottom, passing the cyan primary first. Printing is not completed until all three primaries have passed over the same band on the paper, to allow mixing the primary colors. The drops of ink strike the paper or other substrate and then dry to form dots that, when viewed together, create the permanently printed image. Desired image colors are created by combining drops of ink of the primary colors. The individual dots, typically located on 1/300 inch centers, are not discernable to the naked human eye so that arrays of dots can be printed to form what appear to be solid fields of a desired color.
The fundamental unit of printing area on the paper is commonly referred to as a pixel. The nominal pixel size or spacing is equal to the spacing between nozzles on the print head. The speed of the carriage and the frequency of ejecting drops of ink are controlled by the controller to allow depositing successive drops of ink along a horizontal line having a spacing similar to the vertical spacing of the nozzles on the print head. The paper may thus be considered as a regular array of pixel areas, for example, consisting of 300 pixel areas per inch in both directions.
Alternatively, pixels may be visualized as lying on the nodes of a raster of regularly arranged points in two dimensions. In either case, the pixels are not physically marked on the printing medium other than by dots upon printing. They form a useful convention because they permit an assessment of the dotted image quality actually printed compared to a hypothetical ideal standard pixel array. Since it is the visual appearance of the image that is most important, the use of the pixel location concept also permits comparisons of different methods of forming images using various dot deposition strategies.
An important consideration in printing strategies in an ink jet printing system is the intended printing medium. For example, overhead transparencies (OHT) have less affinity for absorbing ink than does a typical paper. As a result, drops of ink deposited on an OHT tend to bead rather than diffuse, as compared to drops deposited on paper. Additionally, the drops of ink deposited on OHT take longer to dry.
U.S. Pat. No. 4,748,453 (Lin et al.) discloses a method of depositing spots of liquid ink upon selected pixel centers on overhead transparencies so as to prevent the flow of liquid ink from one spot to an overlapping adjacent spot. According to that method, a line of information is printed in at least two passes so as to deposit spots of liquid ink on selected pixel centers in a checkerboard pattern, wherein only diagonally adjacent pixel areas are deposited in the same pass. On the second pass, the complementary checkerboard pattern is deposited, thereby completing deposit of ink on all of the pixels in a desired area.
U.S. Pat. No. 4,617,580 (Miyakawa) is directed to improving color saturation in depositing drops of liquid ink on overhead transparencies, by printing multiple drops on each pixel location, each drop being slightly offset horizontally and/or vertically from an adjacent drop.
Printing on paper, however, presents a different problem. Paper has an affinity for the liquid ink so that substantial absorption and diffusion of each drop of ink generally occurs. On the one hand, diffusion from one drop of ink to a drop that occupies an adjacent pixel area is helpful in achieving color mixing and obtaining a solid appearance. Along a boundary between two adjacent fields of different colors, however, such diffusion results in color bleeding across the boundary, making the boundary appear fuzzy. This is an undesirable result.
Most color ink jet printers form desired image colors by depositing two or more droplets of ink of different primary colors, one over the other, on a selected pixel area of the substrate. The net visual effect is a dot of a secondary color determined by the principles of transmitted or reflected color formation. Overprinting two drops of ink on each pixel area results in high color saturation (200%), but also leads to substantial bleeding across color field boundaries, resulting in poor image quality.
In a symmetric array of pixel areas, it follows from the geometry that diagonally adjacent pixel centers are spaced farther apart than horizontally or vertically adjacent pixel centers, by a factor of the square root of 2. Bleeding among adjacent drops of ink thus can be reduced by printing only on diagonally adjacent pixel areas in a checkerboard pattern. However, checkerboard printing leaves 50% unprinted or "white" area, giving the printed area an undesirable pale appearance.
A need remains for a printing method that can provide good color saturation for a solid appearance in a color image while minimizing bleeding across color field boundaries.